1. Field of the Invention
The present invention relates to a scanning probe microscope, such as an atomic force microscope or a friction force microscope.
2. Description of the Related Art
In an atomic force microscope, a force exerted between a cantilever and a sample induces a mechanical displacement of the cantilever. Generally, the displacement is measured with an optical lever. In this scheme, a focused laser beam is made to hit the back surface of the cantilever. The reflected light is detected by a photodiode that is segmented into four parts, for example. Thus, the displacement of the cantilever is measured.
When no force is exerted between the cantilever and the sample, it is important that the laser light reflected off the cantilever impinge on the proper position of the light-sensitive surface of the photodiode, i.e., its center. An adjustment for this is made by a human operator who adjusts the position of the photodiode while directly viewing the light-sensitive surface of the photodiode such that the laser light hits the center of the surface.
However, the photodiode is small and so it is very difficult to adjust the position in such a way that the laser light hits the center of the surface.